The invention is based on a priority application EP 05291891.9 which is hereby incorporated by reference.
The invention relates to a method for operating a passive optical network which transmits sub-frames of data arranged in a common transmission frame in at least two signalling modes from an optical line termination to a plurality of optical network units, to an optical line termination with means for performing the method, to a passive optical network, and to a corresponding transmission frame structure.
A passive optical network (PON) with dynamic multi-level pulse amplitude modulation (PAM) generating a scheme of multi-level signalling modes (e.g. a binary level and a 4-level mode in downstream) basically transmits at a symbol rate of 12244 or 2488 MSymbols per second (MSps) continuously and switches the signalling mode dynamically in the sub-frames (or packets, respectively) for/from each optical network unit (ONU). Such a PON is commonly provided with a point-to-multipoint structure (tree structure), having an optical line termination (OLT) as a central station and a plurality (often several tens to thousands) of optical network units (ONUs) in downstream which may represent e.g. subscriber stations connected to the OLT via optical fiber lines.
The performance of such a PON can be increased by implementing new signalling modes with an increased number of levels (e.g. 8-level etc.). Although the OLT as a central station may be equipped with relatively small effort with suitable means for generating signals with more levels, due to the excessive number of ONUs in a PON, replacing all receivers which are not specified for processing higher-level signals (referred to as “legacy receivers” in the following) would cause immense costs. Although introduction of burst mode in downstream could be a workaround, if maintaining continuous operation is desired, adding new signalling modes with an increased number of levels must be performed in a downward compatible way, i.e. the transmission signal must contain signal sections which can be processed by the legacy receivers and higher-level sections which the legacy receivers must be able to tolerate without losing sync due to the variable length of those sections.
In the critical downstream path, such multi-level sections would still contain enough signal state transitions to keep the clock data recovery (CDR) of the legacy ONUs in lock. Average transition amplitude is indeed reduced but still sufficient to maintain CDR lock and even full swing transitions are present at a reduced probability. The CDR staying in lock allows ongoing bit-counting and navigation of frames in the transmission convergence (TC) layer even if for the ‘legacy’ ONUs not all data sections are readable.
A typical transmission frame of the TC-layer has often a pre-defined, constant length and consists of a header (physical control block downstream (PCBd)) and several concatenated GEM (Gigabit PON Encapsulation Method) sub-frames of variable length (referred to as GEM frames in the following). To assure continued synchronicity not only in the physical medium-dependent (PHY)-layer (e.g. CDR), but also in the TC layer, it is indispensable that the entire physical control block downstream is transmitted strictly as a 2-level section. For transmission of the subsequent GEM frames, the following approach is known from the state of the art:
The GEM frame headers are always transmitted in 2-level format in order that the GEM headers are readable also for the ‘legacy’ ONUs. Hence sync, HEC, port-ID and length indicators are readable to all ONUs, i.e. also to those having the lowest specification (2-level-only ONUs) which can discard in a qualified manner GEM frames not destined to them. This approach has the disadvantage that the signalling Mode (PAM-mode) has to be switched frequently between GEM headers and GEM payload sections.